Field of Invention
The present invention relates to a field of micro-electro-mechanical systems (MEMS), and in particular, to a capacitive acceleration sensor with a bending elastic beam and a preparation method thereof
Description of Related Arts
MEMS is defined as micro-electro-mechanical systems, which consists of multiple micro mechanism, micro sensors, micro actuators and signal processing, control circuits, communication interfaces, and power supply. This technology originated in microelectronics technology, further widens and extends in the mechanical or mechatronics field. So far, MEMS device mainly includes micro sensors with high accuracy, high efficiency, high reliability and low cost, which are widely used in the fields of information, automotive, medical science, aerospace, defense and the like.
In MEMS sensors, a micro acceleration sensor is an important inertial sensor, and is used as a test device to convert the physical signal of acceleration into an electrical signal to facilitate the measurement. According to different detection methods, MEMS acceleration sensors can be divided into capacitive type, piezoresistive type, piezoelectric type, surface acoustic wave type, tunnel type and the like. Therein a capacitive acceleration sensor with the advantages of high precision, little influence by temperature changes, etc., becomes one of the most widely used acceleration sensors. A capacitive acceleration sensor is provided with a fixed electrode and a movable electrode on a seismic mass, when the mass is displaced by acceleration, the distance between the movable electrode and the fixed electrodes is changed, thereby causing the capacitance value between them to be changed. Via a CN conversion circuit, it is possible to detect the displacement of the seismic mass. Therein, since variable gap is used in a differential force balance acceleration sensor, the micro acceleration sensor may have the advantages of simple structure, fast dynamic response, and enable to achieve non-contact measurement, good sensitivity, high resolution, and the measurement of micro displacements.
A sandwich capacitive acceleration sensor can achieve higher detection accuracy, and the method for preparing the sandwich capacitive acceleration sensors is mainly the method of bulk silicon micro-mechanical machining. In order to improve the detection sensitivity of micro acceleration sensor, various research institutions and companies have provided a variety of methods. Specifically, the methods include increasing the mass of a sensitive seismic mass, controlling the thickness of a cantilever, and using a full symmetrical differential structure.
For example, W. S. Henrion et al. employ a double layer bonded silicon beam method to form a beam-mass structure whose double sides are parallel and symmetrical (cf. Sensors structure with L-shaped spring legs, U.S. Pat. No. 5,652,384), the process thereof may employ a method combining KOH etching with dry deep etching releasing. Namely: firstly, KOH is used to etch the silicon wafer from the back to a thickness of the remaining beam, dry deep etching is then used to release the beam-mass structure from the front; two same beam-mass are further bonded back to back to achieve a dual-side structure. The process is complicated and the cost is comparatively high.
As another example, Gao et al. employ a SOI silicon wafer of the double device layer to prepare a middle layer, and prepare anti-overloading bumps on the upper and lower electrode plates, and then the sandwich structure is formed by performing a bonding process. The disadvantages of such method are that the dimension of sensor chip cannot be effectively used due to the big area occupied by L-shaped beams of the sensor; and the bonding process for preparing anti-overloading bumps on the upper and lower electrode plate is complicated.
As another example, H. Seidel et al. employ the concentrated boron-doped self-stop method (cf. Capacitive Silicon Accelerometer with Highly Symmetrical Design, Sensors and Actuators A: Physical, Vol. 21, pp. 312-315), to prepare the entirely symmetrical beam-mass structure, and during the process of KOH etching the beam-mass structure, the concentrated boron-doped layer serves as the etch stop to decide the thickness of the beam, and also serves as a mask in KOH etching in light doped regions. The disadvantages of such method are that nonuniformity of the doping concentration results in nonuniform thickness of the beam and that the residual stress generated in boron-doping process may affect the performance of the acceleration sensor, such as sensitivity and linearity, etc.